Use new macro debounce-reduce to make mouse scaling of images more responsive

Use new macro debounce-reduce to make mouse scaling of images more responsive

* lisp/emacs-lisp/timer.el (debounce, debounce-reduce): New macros.

* lisp/image.el (image-increase-size, image-decrease-size):
Use funcall to call image--change-size-function.
(image--change-size-function): Move code from defun of
image--change-size to defvar that has the value of lambda
returned from debounce-reduce.  (Bug#38187)